This invention relates to circuits for electronic strobe lights such as are used to provide visual warning in electronic fire alarm devices and other emergency warning devices. Such devices are frequently associated with audible warning devices such as horns, and provide an additional means for getting the attention of persons who may be in danger. The strobe light commonly utilized in such systems in a gaseous discharge tube the firing of which is initiated by a trigger circuit which forms part of the flash unit. Energy for the flash typically is supplied from a capacitor connected in shunt with the flash unit and occurs when the voltage across the flash unit exceeds the threshold firing voltage required to actuate the trigger circuit. After the flashtube is triggered, it becomes conductive and rapidly discharges the shunt capacitor until the voltage across the flashtube has decreased to a value at which the flashtube extinguishes and becomes nonconductive.
Typically, such strobe alarm circuits are designed to be operative whether energized from a DC source or from a full-wave rectified input and include a DC-to-DC converter, an inductor coupled to the capacitor connected in parallel with the flash unit, and a switching circuit for connecting and disconnecting the inductor across the DC source to store energy in the inductor during closed periods of the switch and to transfer stored energy from the inductor to the storage capacitor during open periods of the switch. In most applications, it is necessary to miniaturize the circuitry, including the DC-to-DC converter, to the maximum extent possible so that it can be installed inside the lens of the strobe light. Also, the cost of the circuit desirably is kept as low as possible and at the same time its efficiency and reliability must be as high as possible, even when exposed to extreme environmental conditions.
U.S. Pat. No. 5,121,033, assigned to the same assignee as the present application, describes a strobe circuit exhibiting these properties in which the DC-to-DC converter portion of the circuit utilizes an optocoupler for controlling closing and opening of a switch for repetitively connecting and disconnecting an energy-storing inductor across a DC power source. The light-emitting diode portion of the optocoupler is connected in parallel with a resistor connected in series with the inductor for continuously monitoring the current flowing through the inductor; when the inductor current has attained a value at which the voltage drop across the resistor is sufficient to turn on the LED, the switch portion of the optocoupler is turned on and actuates a switch which disconnects the inductor from across the source. After a short interval determined by the parameters of the optocoupler and associated circuitry, the cycle is repeated.
While this prior circuit limits the peak current flowing through the inductor to a relatively constant predetermined value over a range of variations in amplitude of the supply voltage, there is a need for better regulation of input power and flash rate over the operating input voltage range of the energizing source, whether it is a DC source or a full-wave rectified source.